Replacing one string with another for a set of text files using python file.readlines() and .replace() not saving replacement

Question

I have tried to iterate line by line through the text file to find the sections I'm looking to replace and update them with the desired values. The problem I have is that I seem to be updating the value but the value is not written in to the the string. I have printed the output from the file as an explanation of sorts.

for files in glob.iglob(directory+'*'+suffix, recursive=True):
    if "-pst" not in files and "Static" not in files:
        print(files)
        file1 = open(files,"r")
        data=file1.readlines()
        for line in data:
            if "6168" in line:
                print(line)
                line=line.replace("6168", GuideNodes[0])
                print(line)
            line=line.replace("6158", GuideNodes[1])
        
        for line in data:
            if "6168" in line:
                print(line+"$")

The output for this code is:

 6168, 1

 6170, 1

 6168, 4

 6170, 4

 6168, 2

 6170, 2

 6168, 3

 6170, 3

 6168, 5

 6170, 5

 6168, 6

 6170, 6

 6168, 1
$
 6168, 4
$
 6168, 2
$
 6168, 3
$
 6168, 5
$
 6168, 6
$

So you can see that the code initially updates the line but this is not carried over into "data".

I'm working line by line as I also need to move some lines in the code as the text file is a script for another application otherwise I would just edit the text as one long string using file.read().

I'd greatly appreciate if anyone can explain why this is not working as I'd expect!

Answer 1

Strings in Python are immutable, so when you write line=line.replace("6168", GuideNodes[0]) , you create a new string and assign it to line , but it does not change the original value on your data list.

To update the contents of the list, you should change your inner loop to:

for idx, line in enumerate(data):
    if "6168" in line:
        print(line)
        line=line.replace("6168", GuideNodes[0])
        print(line)
    line=line.replace("6158", GuideNodes[1])
    data[idx] = line   # Replaces the original line in data